NASA Study Connects Southern California, Mexico Faults

A multiyear study has uncovered evidence that a 21-mile-long (34-kilometer-long) section of
a fault links known, longer faults in Southern California and northern Mexico
into a much longer continuous system. The entire system is at least 217 miles (350
kilometers) long.

Knowing how
faults are connected helps scientists understand how stress transfers between
faults. Ultimately, this helps researchers understand whether an earthquake on one
section of a fault would rupture multiple fault sections, resulting in a much
larger earthquake.

A team led by
scientist Andrea Donnellan of NASA's Jet Propulsion Laboratory in Pasadena,
California, recognized that the south end of California's Elsinore fault is
linked to the north end of the Laguna Salada fault system, just north of the international
border with Mexico. The short length of the connecting fault segment, which
they call the Ocotillo section, is consistent with an immature fault zone that
is still developing, where repeated earthquakes have not yet created a
smoother, single fault instead of several strands.

The Ocotillo section
was the site of a magnitude 5.7 aftershock that ruptured on a 5-mile-long (8-kilometer-long)
fault buried under the California desert two months after the 2010 El
Mayor-Cucapah earthquake in Baja California, Mexico. The magnitude 7.2 earthquake
caused severe damage in the Mexican city of Mexicali and was felt throughout
Southern California. It and its aftershocks caused dozens of
faults in the region -- including many not previously identified -- to move.

Seismic activity in the region is a sign of its complex
geology. The Pacific and
North American plates are grinding past each other in Southern California. In
the Gulf of California, there's a spreading zone where plates are moving apart.
"The plate boundary is still sorting itself out,"
Donnellan said.

Donnellan's
team has been studying this region since 2009, using data from NASA's
Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR). This
sophisticated airborne instrument measures the ground level with extreme
accuracy, allowing scientists to see how the ground has shifted between
flights. The team also uses data from GPS stations in the region, which provide
information on vertical motion of the ground. The study included team members
from JPL, the University of California's Irvine and Davis campuses, and Indiana
University.

In the new study, Donnellan's team was also able to better
define where Earth's crust continued slipping or deforming following the El Mayor-Cucapah earthquake
and where other factors are important. "The shaking is only part of the
earthquake process," she said. "The Earth keeps on moving for years
[after the shaking stops]. What's cool about UAVSAR and GPS is that you can see
the rest of the process."